THKEE PERIODS IN THE HISTORY OF ONTOGENY. 49 



from Aristotle to Caspar Friedrich Wolff, to the year 1759, 

 when the Theoria Generationis appeared and laid the 

 foundation for future work. The second, to which we now 

 turn our attention, comprises exactly a century ; that is, 

 to the year 1859, in which appeared Darwin's work on 

 " The Origin of Species," which reformed the whole basis of 

 the science of Biology, and especially of Ontogeny. The 

 beginning of the third division is as recent as the time of 

 Darwin. 



As Wolff's labours remained entirely unnoticed during 

 hall a century — till the year 1812 — we are not quite 

 accurate in assigning the exact duration of a century to the 

 second division. During fifty-three years not one book 

 appeared which followed in the lines laid down by Wolff, 

 and carried on his Theory of Evolution, His opinions, 

 which were perfectly correct and founded directly on actual 

 observations, were only occasionally mentioned, and then 

 only to be rejected as erroneous. His opponents, followers 

 of the prevalent and mistaken theory of Pre-formation, did 

 not even deign to refute him. This was owing, as I have 

 said before, to the extraordinary authority possessed by 

 Albrecht Haller, Wolff's distinguished opponent, and the 

 circumstance furnishes one of the most remarkable examples 

 of the influence which a great authority may, as such, long 

 exert against the clear recognition of facts. The neglect 

 of Wolffs labours was so universal that in the beginning 

 of this century two naturalists, Oken (1806) and Kieser 

 (1810), undertook independent investigations into the 

 development of the intestinal canal in the Chick, and 

 obtained a correct insight into Ontogeny, without being 

 aware of the existence of Wolff's important work in the 



